1. Field of the Invention
This invention relates generally to digital data communications systems. More particularly, this invention relates to a system for bidirectional communication of serial-bit-frame data without protocol information from a serial-access digital control computer, over serial data transmission channels, through control computer software-controlled series-to-parallel station interfaces each having a single multifunctional shift register, and then to peripheral devices.
This invention is particularly useful in digital process control systems, for example, to communicate data accurately in the severe physical and noisy electrical environments of an automated steel rolling mill as disclosed below.
2. Description of the Prior Art
Recent increases in demands for more production and higher quality products from the steel industry have prompted changes in mill installations. For example, in automated steel rolling mills, mill speed has been increased and control emphasis has shifted from simple local analog control systems to complex digital process control systems to produce closer and more accurate mill control. In the latter type of system, a programmed digital control computer is the central system component and it is involved in data communications to peripheral devices over lines hundreds and sometimes thousands of feet long. Peripheral devices include: parameter sensors and actuators on the rolling mill, presets, controllers, displays, recording terminals, programmers' desk and a remote scheduling desk.
A high degree of accuracy and reliability of data communications to and from the computer is necessary to maintain the desired high degree of rolling mill control. This is a must despite the fact that data traverses great distances and data communications, as well as other system components, are subjected to the severe physical and noisy electrical environment prevailing in steel rolling mills.
One prior art approach to the data communication problems was to convert each analog sensor and actuator signal to corresponding binary digital signals at the rolling mill site and transmit the digital signals to and from a control computer having a comprehensive parallel-access input-output section and a memory section organized for parallel storage and comparison purposes. Individual multiconductor cables extended parallel data communications from the computer to each peripheral device named above. This resulted in a proliferation of data cables which proved rather costly from procurement, installation and maintenance viewpoints. The first two costs have been known to approximate forty percent of the initial mill automation costs. In addition, add-on capability of this system was also expensive and sometimes unattainable in the computer without significant modifications to its input-output section.
Another approach to the data communications problems was to use a commercially available serial-access control computer and serial looping data communication cables interconnecting the computer with peripheral devices by way of serial station interfaces. Two general types of serial data communications equipment are available and are described as follows.
The first type of serial communication equipment includes a high frequency coaxial cable used for data communications to and from interface devices. Each interface has an R.F. carrier modem and complex hardwired logic elements for decoding protocol information, such as station code number, synch. bit, demand access, polling and others, in addition to decoding control data information. This equipment while suitable for some installations generally will not operate reliably in steel mill environments because of analog problems of adjustment and tuning. In addition, it is slow, expensive, complex and difficult to maintain.
The second type of serial data communications equipments is adapted for point scanning and includes a multiconductor cable having up to 96 lines for transmitting both data and protocol information in parallel to and from interface devices. Each interface eliminates modems, but has very complex hardwired logic elements for decoding protocol as well as control data information. This equipment is also slow, expensive, complex and difficult to maintain.